Most organic materials coated or deposited in thin films are, at some stage in their manufacture, crystalline compounds. Crystalline materials can exhibit a phenomenon referred to as “polymorphism,” the ability of a substance to crystallize in more than one crystal structure formation. Over a quarter million such examples have been reported in the literature (J. Bernstein, J. Phys. D: Appl. Phys., 26, B66-B76, 1993). Each modification or “polymorph” has the same chemical structure, but differs in the packing of molecules within the crystal lattice. These different polymorphs can have profoundly different physical or electronic properties, including color, optical properties, melting point, reactivity, solubility, density, and toxicity (T. L. Threlfall, Analyst, 120, 2435-2460, 1995). While most of the literature relating to polymorphs has focused on pharmaceutical products (S. R. Byrn, Solid State Chemistry of Drugs, Academic Press, New York, 1982), it is equally important in the design or manufacture of thin films and thin film-containing devices to be able to accurately characterize both the incorporated state or phase of such organic materials as well as the presence or absence of other organic species. Unfortunately, the processes by which such films are created can often alter or determine the final phase of the organic materials in the thin films, and so it is very desirable to identify a process by which the phase of these organic materials in such thin films or thin film-containing devices can be characterized.
The presence or absence of many organic materials in a thin film or thin film-containing device may be determined by conventional analytical techniques, such as high performance liquid chromatography or, for polymorphic phases in particular, hot stage microscopy (W. C. McCrone in Physics and Chemistry of the Organic Solid State, D. Fox, M. M. Labes and A. Weissberger, eds, Interscience, New York, 1965, vol. II, p. 725). But such techniques require destructive sample preparation, which can be both time-consuming and unsuitable for a manufacturing environment. Further, the sample preparation process can alter or obscure the phase of the organic materials of interest.
The most commonly used technique for identifying and distinguishing polymorphic materials is X-ray diffraction (XRD) (H. Takahashi, T. Takenishi and N. Nagashima, Bull. Chem. Soc. Japan, 35, 923, 1962). An XRD single crystal structure determination, although potentially time-consuming, can reveal the precise molecular structure and unique crystal habitat of the material. However, XRD single crystal determination is not suitable for use in thin films as it requires a single crystal oriented in the X-ray beam, not an array of randomly oriented crystals as may be found in a film. The more easily attainable XRD powder pattern, obtained from a randomly-arrayed mass of crystals, does not generally provide direct structural information, but its characteristic pattern of diffraction intensity as a function of diffraction angle (the two-theta angle, measured in degrees) can provide a multiplicity of peaks that taken in combination can uniquely and unequivocally distinguish one polymorphic form from others. Unfortunately, when coated as or in a thin film, many polymorphs can appear to be amorphous in nature, that is, failing to show the XRD diffraction pattern expected for a crystalline material. This occurs because some polymorphic materials in the thin films lack the long-range order of isolated crystals that is detectable by XRD, though they maintain a short-range order, which determines their polymorphic character.
Even for those materials that may exhibit a diffraction pattern as or in a thin film, an additional problem may occur when these films are part of a thin film-coated device. These devices, such as an organic electroluminescent (EL) device (C. H. Chen, J. Shi and C. W. Tang, Macromol. Symp. 125, 1-48, 1997), typically are constructed with multiple layers, one or more of which may attenuate or be impermeable to X-rays, thus making the characterization of polymorphs in a thin film incorporated as part of such a device extremely difficult or impossible by the XRD technique alone.
Vibrational spectroscopy, in particular Raman spectroscopy or infrared (IR) spectroscopy (N. B. Colthup, L. H. Daly and S. E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 2nd ed. 1975) can be used to distinguish one polymorphic material from another in a thin film (though commonly this has been limited to thin films between salt or KBr plates, see M. Kuhnert-Brandstatter and E. Junger, Spectrochim Acta, Part A, 23, 1453, 1976) or in a thin film-device but only if the polymorph has been previously characterized. Vibrational spectroscopy can detect the short-range order of a polymorph in a thin film, but cannot in itself fully characterize the molecular structure or crystal habit of the material.
Doff, et. al. have compared separately the utility of X-ray diffraction and infrared spectroscopy in quantitative determination of the polymorphs of alpha and beta inosine, but these measurements were not in a thin film or thin film-device and required extensive sample preparation and manipulation (D. H. Doff, F. L. Brownen and O. I. Corrigan, Analyst, 111, 179-182, 1986). In the preparation of a desired crystalline form of the drug ranitidine, Crookes used separately the infrared spectrum of an oil mull preparation of the drug and an X-ray powder diffraction of the drug to identify the desired form for patent claims (D. L. Crookes, GB 2084580A, Apr. 15, 1982).
Another technique, microfluorescence can be used to distinguish previously characterized polymorphs in a thin film, but only if the materials of interest exhibit a fluorescence spectrum and that spectrum is sufficiently distinct from the fluorescence spectra of other materials in the thin film or thin film-device. A further technique, ellipsometry, which uses polarized light to probe the dielectric properties of thin films can exhibit a sensitivity to optical absorption which can interfere with measurements and may require a detailed knowledge of the real and imaginary parts of the substrate refractive index which may not be available. For some problems, such as the dopants used in small quantities in the thin films of organic electroluminescent (EL) devices, ellipsometry may be insufficiently quantitative.
It is a problem to be solved to provide a non-destructive process for characterizing the polymorphic character or phase of an organic material as a thin film or incorporated in a thin film as well as the presence or absence of other organic materials. It is a further problem to be solved to characterize such phases when the thin film is shielded by an X-ray attenuating medium as when the thin film is incorporated into an electronic device, such as an organic electroluminescent (EL) device. It is a further problem to be solved to be able to quantify the absolute or relative amounts or coverage of such phases in a thin film or a thin film-containing device. Additionally, it is a problem to be solved to determine the thickness of a thin film of an organic material. It is a further problem to be solved to make these determinations of phase, amounts, and film thickness by a non-destructive technique applicable in a manufacturing environment.